Centrosomes

Immediately outside the nucleus are one, or frequently two, small bodies (centrosomes) forming the centres of spherical masses of densely-packed radiating protoplasmic granules. These spherical masses are known as attraction spheres; they are seen best in cells which are undergoing division.

Properties of Protoplasm - The chief characteristics of protoplasm are its powers of motion, nutrition, and reproduction. It possesses also the remarkable faculties of storing past impressions and of making provision for the future.

Within the cell, in many cases, a streaming movement of the protoplasm can be demonstrated; some surface cells, such as those of the respiratory passages, possess hair-like processes (cilia) which are constantly in motion so as to keep the surface free of deposited materials; but one of the most striking exhibitions of motile power is that presented by the white corpuscles of the blood.

These corpuscles resemble greatly the unicellular organism, the amoeba, and hence their movements are described as amoeboid: a portion of the proto-plasm is protruded from the cell as a finger-like process (pseudopodium) into which the rest of the mass is gradually drawn, and the whole cell thus shifts its position. Associated with the protoplasmic movements other forms of energy are liberated which manifest themselves in electrical changes, chemical changes, heat, and occasionally light. The initiation of any or all of these changes is due to some alteration in the environment of the animal or cell; this alteration, which may be either chemical, mechanical, thermal, or photic, is known as the 'stimulus,' while the change in the cell is called the 'response.'

In order to maintain its energy, protoplasm has the power of selecting and taking up inert, nutritive substances and converting them into its own living material. As a necessary corollary to this it has also the power of extruding waste products resulting from the various chemical decompositions which accompany the manifestations of life. In multicellular organisms certain cells are specialised to act as manufactories for substances (secretions) required by the other cells, whilst other groups of cells take over and deal with the waste products of the organism (excretions).

All living cells are capable of reproducing themselves by subdivision. Only in a few instances is the subdivision effected by a simple cleavage or by budding off. In most cases a complex series of changes takes place, originating in the nucleus and centrosomes and later involving the cytoplasm; this process is known as indirect division, mitosis, or karyokinesis (Fig. 3). The nuclear membrane and the nucleolus disappear and the chromatin of the nucleus arranges itself in a spiral. The spiral soon breaks up into a fixed number of V-shaped portions which collect at the central zone or equator of the nucleus. Each V-shaped mass splits longitudinally into two and the resulting parts retreat to the two poles of the nucleus - an equal number to each - where they rearrange themselves again into spirals. A new nuclear membrane forms around the spiral and a new nucleolus appears. The cytoplasm becomes constricted opposite the equator of the original nucleus and by the deepening of this constriction the cell is ultimately split into two.